Einsteins two postulates First postulate(the relativity principle) The laws of physics have the same form in all inertial reference frames Second postulate(constancy of the speed of light) Light propagates through empty space with a definite speed c independent of the speed of the source or observer Give up commonsense notions of space and time
Einstein’s two postulates 6 First postulate (the relativity principle): The laws of physics have the same form in all inertial reference frames. Second postulate (constancy of the speed of light): Light propagates through empty space with a definite speed c independent of the speed of the source or observer. Give up commonsense notions of space and time!
Simultaneity Time is no longer an absolute quantity! Simultaneity of events depends on the observer Same time at same place /timing by light A BB (a)Observer o(b)Observer O
Simultaneity 7 Time is no longer an absolute quantity! Simultaneity of events depends on the observer A1 B1 O1 . A2 O2 B2 . v (a) Observer O1 (b) Observer O2 Same time at same place / timing by light
Time dilation(1) Mirror Mirror MOTION D Event 1 Event 2 Event I Event n B (b)Observer C on Earth 2D (a Observer B on spaceship
Time dilation (1) 8 (a) Observer B on spaceship (b) Observer C on Earth 0 2D t c =
Time dilation(2) Mirror Mirror MOTION D Event I Event 2 Event I Event 2 B B 2D 2L2v△t △t C C 2/×/c1)2 △t= >△ 1/C
Time dilation (2) 9 0 2 , D t c = 2L t c = 2 2 0 2 2 2 v t c t c = + 0 0 2 2 1 / t t t v c = −
Time dilation(3) △t △t v/C Clocks moving relative to an observer are measured by that observer to run more slowly(as compared to clocks at rest) 1)Proper time(events occur at same point 2)Relativity factor y=1//C2 3)Relativistic effect universalit 4)Space travel twin paradox 10
Time dilation (3) 10 Clocks moving relative to an observer are measured by that observer to run more slowly (as compared to clocks at rest). 0 0 0 2 2 2 1 / 1 t t t t v c = = = − − 2) Relativity factor 3) Relativistic effect & universality 4) Space travel & Twin paradox 1) Proper time (events occur at same point) 2 2 = − 1/ 1 / v c